Conventional continuous digesters utilize a feed system, including a high pressure feeder, for feeding a slurry of chips in cooking liquor either directly to the top of a continuous digester, or through an impregnation vessel and then to the top of the digester. Most designs of conventional Kamyr.RTM. continuous digesters are provided to allow the cooking of softwood chips, the softwood chips containing substantial amounts of lignin. Because substantial cooking of softwood chips is necessary, the continuous digester includes a cooking loop circulation, as well as other circulations.
Hardwood chips contain less lignin than softwood chips and thus it is easier to cook them, requiring less time at temperature to dissolve and remove the lignin. According to the present invention, an apparatus and method are provided which specifically take into account the fact that hardwood chips are easier to cook, which has a number of advantages over conventional systems. According to the invention a separate impregnation vessel is unnecessary, the digester cooking circulation with associated pumps, heaters, and piping are eliminated, and a more uniform heating of the chip mass and subsequent better quality pulp may be obtained.
According to one aspect of the present invention, a method of producing chemical pulp from hardwood chips utilizing a vertical digester having a top and a bottom, and a high pressure feeder, is provided. The method comprises the steps of continuously and substantially sequentially: (a) Steaming the hardwood chips. (b) Entraining the hardwood chips in liquor at a relatively cool temperature, so that the liquor will not significantly flash into steam in a high pressure feeder. (c) Feeding the chips entrained in relatively cool liquor under high pressure toward the top of the digester using the high pressure feeder. (d) Externally of the digester, replacing the relatively cool liquor entraining the chips with relatively hot cooking liquor, at cooking temperature, and recirculating the replaced relatively cool liquor to the high pressure feeder, so as to significantly thermally isolate the high pressure feeder from the digester. (e) Feeding the hardwood chips entrained in cooking temperature cooking liquor to the top of the digester. (f) At the top of the digester, separating some liquor from the chips. (g) Recirculating and reheating the liquor separated from the chips in step (f), to provide a significant part of the cooking liquor in step (d). And, (h) cooking the chips in the digester in a cooking zone, to ultimately produce chemical hardwood pulp, and removing the hardwood pulp from the bottom of the digester.
Steps (b) and (c) are practiced with cooking liquor at a temperature of roughly 240.degree. F., and steps (d) and (g) are practiced with cooking liquor at roughly 290.degree. F., for kraft cooking. Cooking liquor is added to the liquor separated during the practice of step (f), prior to the reheating in step (g). Step (h) is preferably practiced without the addition of heat during the cooking zone, utilizing only heat provided from the cooking liquor added in step (d). There is also typically the further step of, after step (h), further cooking the chips in a modified cooking zone, and washing the chips in the digester, and withdrawing, heating, and recirculating liquors in the modified cooking and washing zones. Step (d) is typically practiced utilizing an inverted top separator.
The invention also comprises apparatus for cooking hardwood chips. The apparatus comprises the following components: A steaming vessel. A high pressure feeder. A chips chute operatively connecting the high pressure feeder to the steaming vessel. A vertical digester having a top and a bottom. First separating means, external of the digester, for replacing a first liquor entraining chips with a second liquor, the separating means disposed between the high pressure feeder and the top of the digester. Second separating means, internal of the digester at the top thereof, for withdrawing some of the liquor entraining chips introduced into the top of the digester. First recirculating means for recirculating the first liquor from the first separating means to the high pressure feeder. Second recirculating means for recirculating liquor separated by the second separating means to the first separating means, the second recirculating means including a heater. And, pulp withdrawal means for withdrawing pulp from the bottom of the digester.
The first separating means preferably comprises an inverted top separator, while the second separation means comprises a continuous conical screen disposed in the interior of the digester at the top thereof. The vertical digester has a cooking zone, and preferably the digester is devoid of a recirculation loop in the cooking zone, although recirculatory loops are provided associated with modified cooking zones and wash zones further down in the digester, if present.
It is the primary object of the present invention to provide a method and apparatus for simply yet effectively cooking hardwood chips to produce chemical pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.